Dehydrogenation of hydrocarbon gases is carried out at a high temperature of at least 550° C. Because the catalytic reaction occurs at high temperature, it is accompanied by side reactions such as thermal decomposition and coke formation. The extent of such side reactions acts as an important factor that determines the selectivity and the activity of catalyst. Among the side reactions, the coke formation reaction causes the catalytic active material to be covered with coke which prevents contact with a reactant, undesirably decreasing the total reaction conversion. Furthermore, as the coke formation progresses, the inlets of pores of the catalyst are blocked, so that the active material present in the pores is rendered useless, drastically accelerating the deactivation of the catalyst.
In addition, the dehydrogenation catalyst for the hydrocarbons is required to be thermally stable. Because of the high reaction temperature and heat generated during the regeneration of coked catalyst, thermal deformation of catalyst itself and structural sintering may result, thus causing changes in the catalytic reactivity. For this reason, structural compatibility of the catalyst, thermal stability of catalyst structure, thermal stability of an active component, and regeneration of the coked catalyst are regarded as important when making the determination of a superior catalyst.
Typically, dehydrogenation catalysts are classified into chromium oxide catalysts and platinum catalysts.
In a chromium-based catalyst (U.S. Pat. No. 6,797,850), the deactivation rate of the catalyst is fast attributed to coke formation and thus the regeneration rate is also fast, so that the lifetime of the catalyst is shorter than that of a platinum-based catalyst, and there are problems due to the toxicity of chromium itself.
Exemplary platinum-based catalysts are a catalyst having an outer layer containing an active component of 40˜160 μm, and a layered catalyst including gamma-alumina (U.S. Pat. No. 6,756,515) or alpha-alumina (U.S. Pat. No. 6,486,370) as an inner layer, but the inner layer that defines the specific surface area of the catalyst by pores has no metal active component resulting in low dispersibility and low active area. Furthermore, when gamma-alumina is used, side reactions may increase due to acid sites of alumina itself, and changes in structural properties in which alumina crystallinity changes and the specific surface area decreases may take place during the reaction. On the other hand, alpha-alumina may decrease dispersibility of noble metals due to the low specific surface area and may reduce the entire active area of a noble metal, leading to low catalytic activity.
For the preparation of catalysts, there is a disclosed a platinum-based catalyst having no chlorine which is applied to the dehydrogenation of ethane (U.S. Pat. No. 7,375,049). When chlorine is not contained in this way, initial activity of the reaction may be high. However, in the case where this catalyst is used for a long period of time to carry out the process, the active metal component may be sintered, and thus dispersibility may decrease, undesirably deteriorating catalytic activity (Catalysis Today 111 (2006) 133-139).
The platinum-based catalysts are prepared using silica (U.S. Pat. No. 7,432,406), zeolite or boron silicate (U.S. Pat. No. 6,555,724) as a carrier thereof, but these catalysts are composed mainly of pores having an average pore diameter of 10 nm or less, and thereby very sensitively acts for structural closure attributed to coke formation, undesirably drastically deactivating the catalyst.
Conventional dehydrogenation catalyst related patents include contents regarding kinds of active components and carriers of catalysts, and pore distribution which is one of the physical properties of catalysts has not yet been introduced. The pore volume and the pore size are important factors that determine the material transfer coefficient of reactants and products, and the diffusion resistance of a material under conditions of a rapid chemical reaction rate determines the total reaction rate, and thus a structure having large pores may be favorable in terms of keeping the activity of the catalyst high, and the use of a carrier having a large pore size makes it difficult to stack coke and is thus favorable in maintaining the activity of the catalyst.
Therefore, the development of a dehydrogenation catalyst having a macropore size and being superior in activity, selectivity and coke stability is required.